Composite Curve Number Calculator with Unconnected Impervious Area
Calculate the composite curve number for a watershed that has an unconnected impervious area using:
\[
CN_{comp} = f_{UIA} \times CN_{UIA} + \left(1 – f_{UIA}\right) \times CN_{perv}
\]
* Enter the percentage of unconnected impervious area, the curve number for that area, and the curve number for the pervious area.
Step 1: Enter Watershed Data
Example: 30% of the watershed is unconnected impervious
Example: 75 (typical value for unconnected impervious surfaces)
Example: 70 (typical for pervious areas)
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